Meteorite Dust Hints at Solar System's Origins

By Michael Schirber |
July 30, 2009 12:52pm ET

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Where do we come from? The answer
varies depending on how far back you want to look. Researchers are studying the
oldest meteorite grains to figure out the origin of our solar system. Some of
the planet-making material may have resulted from another galaxy smacking into
ours.

Around 4.6 billion years ago, our
yellow star and its planet-filled disk arose out of a dense molecular cloud.
Most of the details about this pre-solar environment were lost to heating of
the primordial gas and dust, but some
rocky grains escaped alteration and therefore preserve clues of our solar
system's distant past.

"Presolar
grains are the oldest materials in the solar system," says Philipp Heck of the University of Chicago. "The ages
of the grains clearly indicate that they are older than the solar system."

But just how old?

Heck and his colleagues isolated 22
grains from the Murchison
meteorite, which is well-known for the organic material it contains, and
measured how long the grains spent in interstellar space before winding up in
our nascent solar system. The implied grain ages, reported in a recent paper of
the Astrophysical Journal, appear to support a hypothesis that our solar
system formed after a smaller satellite galaxy crashed into the Milky Way
around 6 billion years ago.

Pre-solar graininess

Scientists have known since the
1960s that meteorites contain
rare isotopes (an isotope is a more or less massive version of an element).
In 1987, some of these isotopes were found to be localized in micron-sized
grains that presumably formed outside of our solar system and somehow survived
the planet-formation process.

For example, some small diamond
crystals in meteorites have high levels of heavy xenon isotopes that imply that
they originated from a supernova
explosion.

"Isotopic abundances tell us
what type of star a pre-solar grain came from or which planet or parent body a
meteorite comes from," Heck says.

Heck and his colleagues looked in
particular at silicon carbide grains from the Murchison meteorite. These grains
stand out by having more heavy silicon isotopes than anything else in the solar
system. In particular, the levels of silicon-29 and silicon-30 (relative to the
more common silicon-28) are as much 100 times that found in normal silicon
crystals.

Because of these anomalous
abundances, scientists believe that pre-solar silicon carbide grains formed in
the winds blowing off of asymptotic giant branch (AGB) stars, which are often
called "carbon" stars because of the high levels of carbon detected
in their atmospheres.

Interstellar travel times

The silicon carbide grains also
contain other isotopes that speak of the long interstellar voyage the grains
made from their carbon star sources to the molecular cloud that eventually
became our solar system.

The interstellar medium is full of
cosmic rays, which are highly energetic protons and other small nuclei. When a
cosmic ray smashes into a space-faring grain, it can break up one of the
resident atoms into several pieces. Some of these pieces are isotopes that are
not expected to form in stars, Heck explains.

The so-called "cosmogenic" isotopes are like a clock ? the more of
them that a grain contains, the longer it spent in interstellar space being
bombarded by cosmic rays. (Once the grain becomes incorporated into the larger
space rocks of a burgeoning solar system, it is no longer exposed to as many
cosmic rays, and therefore the clock stops.)

Other researchers have tried to
measure this interstellar travel time, but their results have been plagued by
uncertainties over what exactly happens to atoms that are struck by cosmic
rays.

Heck and his colleagues use an
improved model for cosmic ray interactions, and they focus on chemically inert
gases, which provide a cleaner record of the past. Using a specially-built
noble gas mass spectrometer, the researchers recorded the amount of helium-3
and neon-21 in their samples. Both of these rare isotopes are created largely
by cosmic ray collisions.

From the isotope abundances, the
researchers estimate that the majority of grains spent between 3 and 200
million years in interstellar space before falling into our molecular cloud
some 4.6 billion years ago.

Galactic smashup

The grain ages are shorter than
expected. Previous studies had estimated that silicon carbide grains could last
in interstellar space for 500 million years before being destroyed, most likely
by a supernova shock wave.

The fact that almost none of the grains
in Heck's study had pre-solar ages more than 200 million years seems to imply
that something dramatic happened in the galaxy's
history right before our solar system formed.

This dramatic event could have been
a merger between our Milky Way galaxy and a metal-poor satellite galaxy. Such a
collision, proposed by Donald Clayton of Clemson University, would have
unleashed a burst of star formation.

"The merger-induced starburst
about 2 billion years prior to the sun's birth would indeed seem to explain
this data," says Clayton, who was not one of the authors.

The timing of the merger would
explain the lack of "old" silicon carbide grains. It takes about a
billion and a half years for an intermediate-mass star to mature into an AGB
star, so there would be a relative lull in silicon carbide grain production
following the burst in star formation. Then about 200 million years before the
solar system formed, the merger-induced AGB stars started pumping out grains.

The grains didn't stay in one place,
but spread out across the galaxy. In fact, there is evidence that all the
matter that made up our solar system was widely dispersed, says outside
researcher Anthony Jones from the Institutd'AstrophysiqueSpatiale at the
University of Paris.

"We weren't born anywhere
special," Jones says.

However, we may have been born in a
special time. Prior to the merger, our galaxy may not have had enough of
star-processed materials to produce our type of solar system.

"Such a starburst would have
been important, in particular, to deliver carbon and other elements to our
solar system," Heck says.

Michael Schirber has authored a long series of articles about environmental technology. Over the years, he has written for Astrobiology Magazine, Science, Physics World, Live Science and New Scientist.